1. Field of the Invention
The present invention relates to a method of modifying the surface of a plastic substrate used as a substrate for recording medium, an electric component, or a garniture.
2. Description of Background Art
In general, as a substrate of information recording medium such as a compact disk (CD), a videodisk, and an optical memory, plastic substrates made of a polycarbonate resin, a polymethyl-methacrylate (PMMA) resin, an acrylonitrile-butadiene-styrene (ABS) resin, a polyolefin resin or the like are widely used. The information recording medium is produced by forming a layer of a metal such as aluminum, tellurium, and rare earth metallic compound on a plastic substrate made of, for example a polycarbonate resin by a physical deposition method or by forming a layer of a metal by a chemical reaction such as a silver mirror reaction. Information is recorded and reproduced by irradiating a laser beam thereon and detecting a time variation of the intensity of a reflected light. Further, an optical recording medium for recording and reproducing information, which is formed by spin-coating a dye of changing its color by irradiation of light instead of adopting a metallic layer, is used.
In a case of a plastic substrate, because the adhesion strength and the durability of a metallic layer or an organic dye layer to be formed on the substrate are important factors, a method of modifying a surface of the plastic substrate or a method of cleaning the plastic substrate for enhancing the adhesion and the durability have been researched.
As the method of modifying the surface, a plasma treatment under a vacuum atmosphere is proposed. Although the plasma treatment is to modify a surface using ions, electrons, and radicals generated by electric discharge from nitrogen, oxygen, fluorine, argon, or the like in vacuo, electrodes and ions having high energy are apt to destroy the surface of the resin and to soften the low molecular structure, whereby the adhesion strength with the formed layer and the impact strength are decreased. Therefore, use of only radicals is required. Since high energy electrons, ions, and radicals of a discharge gas to be used are simultaneously generated in the discharge gas, a method of separating only the radicals has been sought. It has been proposed to separate radicals by a method of modifying a surface by increasing a distance for irradiating a light from a discharge area utilizing a difference of the lifetimes of electrodes and ions and the lifetime of radicals or by a method of applying an appropriate bias. However, these methods are scarcely used because the treatment is conducted under a vacuum atmosphere, which is not suitable for a practical application and requires a high running cost.
Therefore, ozonation by irradiating ultrasonic wave (UV) is used to contribute adhesion of a layer to be formed. However, it is impossible to selectively remove alkali and halogen components, a problem of corrosion resistance is not solved. Further, it is difficult to remove dust to be deposited.
Since a plastic substrate is easily charged when handling, it is cleaned by applying an ultrasonic wave or the like in a surfactant, an acid solution, or an alkaline solution. This method is effective for removing extraneous matters such as dust to some extent when a reproducing density is low. However, the object of the cleaning is not modification of a surface of resin. Therefore, there are many cases where alkaline and halogen components contained in a catalyst are not sufficiently removed, whereby there is a problem about durability of a metal and a recording dye layer. As for compact disks, the lifetime has been estimated at several decades. However, there are many cases where a light reflectance is deteriorated in about five years so as to make recording and reproducing difficult. Further, there is a case where a metallic film is deteriorated to be a transparent body made of oxide or hydroxide, whereby a missing bit may be generated and the reproducing sound quality may be deteriorated.
As for polycarbonate resins widely used for compact disks, the following mechanism seems to be a reason for deteriorating recording and reproducing characteristics. The polycarbonate resins are ordinarily prepared as products by copolymerizing bisphenol A by an ester interchange method or a phosgene method to make it a pellet resin of high polymer, providing with press molding and cleaning steps, adhering a metallic film. For the copolymerization, a compound containing a metal alcoholate of alkaline metal or alkaline earth metal of Na, K, Be, Mg and so on or containing an alkaline metal group such as Na salt of Na phenolate or dihydroxyallylalkane or the like is widely used as a catalyst. These alkaline catalysts deteriorate a resin when it is left in the resin. Therefore, after the reaction is completed these alkaline catalysts are neutralized by an acid having a high boiling point such as P-toluenesulfonic acid, organic halide, and dialkyl hydrosulfate. However, this neutralizing and removing process is not always perfect on an industrial aspect, wherein a minute unreacted catalyst may be left on a surface of resin to cause deterioration of characteristics of a recording medium.
As for PMMA methacrylate resin, a polyacrylic soda or a sodium sulfate is used in producing a pellet resin. A sodium resinate is used for an ABS resin. A halogen salt such as a metallocene catalyst is used for a polyolefin resin. Phenomenon similar to described above occur also in these cases.
When the recording will be switched to high-density recording using a blue laser beam, or when a resin substrate such as polycarbonate will be used as a substrate for digital recording medium of HDD, resolution of these problems will be strongly required for recording digital information. Although musical information and image information are recorded in digital signals, because the information itself has continuity, a missing bit can be corrected by an interleave interpolation method, whereby the resolution of these problems has not been required in past.
Because digital information used in an ordinary HDD has not continuity to cause bit errors, further reliability different from that in a conventional technique becomes required in recording mediums.